Looking for a recommendation of resources in regard to the topic of ecumenicalism and these three groups.
Catholic, Protestant and Orthodox Theological Discussion Group
In order to understand the Catholic Church's own understanding of ecumenism, I recommend beginning with https://www.logos.com/product/27553/ut-unum-sint.
“God watches over the affairs of those who truly love him without their worrying about them.” - St. John of the Cross
Most of the materials that come to mind are online. Unfortunately, I am aware of only a small sliver to the information available. You can find useful references in:
Orthodox Bishop Hilarion Alfeyev: "To be a theologian means to have experience of a personal encounter with God through prayer and worship."